H2-receptor antagonist

H₂ antagonists, also called H₂ blockers, are a class of medications that block the action of histamine at the histamine H₂ receptors of the parietal cells in the stomach. This decreases the production of stomach acid. H₂ antagonists can be used in the treatment of dyspepsia, but have been surpassed by the more effectiveproton pump inhibitors. They are also used to treat peptic ulcer disease and gastroesophageal reflux disease.

H₂ antagonists are a type of antihistamine, although in common use the term "antihistamine" is often reserved for H₁ antagonists, which relieve allergic reactions. Like the H₁ antagonists, some H₂ antagonists function as inverse agonists rather than receptor antagonists, due to the constitutive activity of these receptors.

The prototypical H₂ antagonist, called cimetidine, was developed by Sir James Black at Smith, Kline & French (now GlaxoSmithKline) in the mid-to-late 1960s. It was first marketed in 1976 and sold under the trade name Tagamet, which became the first blockbuster drug. The use of quantitative structure-activity relationships (QSAR) led to the development of other agents—starting with ranitidine, first sold as Zantac, which has fewer adverse effects and drug interactions and is more potent.